Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. In addition, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (Plug-in HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuels.
Small-sized mobile devices use one or several battery cells for each device. On the other hand, middle or large-sized devices, such as vehicles, use a middle or large-sized battery module having a plurality of battery cells electrically connected to each other because high output and large capacity are necessary for the middle or large-sized devices.
Preferably, a middle or large-sized battery module is manufactured so as to have as small a size and weight as possible. For this reason, a prismatic battery or a pouch-shaped battery, which can be stacked with high integration and has a small weight to capacity ratio, is usually used as a battery cell (unit battery) of the middle or large-sized battery module. In particular, much interest is currently focused on the pouch-shaped battery, which uses an aluminum laminate sheet as a sheathing member, because the pouch-shaped battery is lightweight, the manufacturing cost of the pouch-shaped battery is low, and it is easy to modify the shape of the pouch-shaped battery.
Battery cells constituting such a middle or large-sized battery module may be secondary batteries which can be charged and discharged. As heat is generated from the batteries due to internal resistance of the batteries during charge and discharge of the batteries, temperature of the batteries increases. In particular, during discharge of the batteries, a larger amount of heat is generated from the batteries due to heat caused by exothermic reaction in the batteries with the result temperature of the batteries further increases. With increase in temperature of the batteries, lifespan characteristics of the batteries are deteriorated and gas is generated due to negative reaction. For these reasons, it is very important to cool the batteries.
Furthermore, the laminate sheet of each pouch-shaped battery widely used in the battery module has a polymer material exhibiting low thermal conductivity coated on the surface thereof with the result that it is difficult to effectively lower overall temperature of the battery cells.
Particularly, in a case in which high current is used to provide high output as in an electric vehicle, an amount of heat generated from the batteries further increases. If the heat, generated from the battery module during charge and discharge of the battery module, is not effectively removed from the battery module, the heat accumulates in the battery module with the result that deterioration of the battery module is accelerated. According to circumstances, the battery module may catch fire or explode. For this reason, a battery pack, which is a high-output, large-capacity battery, needs a cooling system to cool battery cells mounted therein.
In recent years, the capacity of an individual battery has been increased to increase overall capacity of a battery pack and reduce cost of the battery pack. As a result, generation of heat from the battery becomes serious.
In order to solve the above problem, a direct air cooling method, which is ineffective, an indirect air cooling method using a heat dissipation plate which exhibits high efficiency but incurs much cost, and a water cooling method to perform cooling using a coolant flowing through a water flow channel formed in a heat dissipation plate.
In a case in which a method performed based on heat transfer to the outside of the battery through convection or conduction is used, however, it is not easy for heat generated from the middle of the battery cell to be transferred to the heat dissipation plate disposed at the outside of the battery cell if the thickness of the battery is increased with the result that it is not possible to uniformly lower overall temperature of the battery cell.
In order to solve the above problem, it is necessary to manufacture a cooling system, which involves great cost. As a result, manufacturing cost of the battery pack is increased.
In addition, when an internal short circuit occurs due to penetration of a needle type conductor into the battery, it is not possible to uniformly restrain the increase in temperature of the battery. Furthermore, the strength of the battery against external impact is low with the result that it is difficult to secure safety of the battery.
Consequently, there is a high necessity for a battery cell that is capable of efficiently removing heat generated from the battery cell during charge and discharge of the battery cell while providing high-output, large-capacity power and that is capable of more efficiently performing a cooling process when an internal short circuit occurs doe to external impact and penetration of a needle type conductor into the battery cell, whereby safety and lifespan characteristics of the battery cell are improved.